1. Field of the Invention
The invention concerns an optic fiber correlator used to give an electrical signal that expresses the correlation of two electrical signals to be correlated, for example, a pulse received by a radar and a reference pulse.
2. Description of the Prior Art
In radar, there are known methods for correlating two signals with a delay of about 10 to 100 nanoseconds, using logic gates and delay lines. However, this method becomes extremely complex for delays of about 10 nanoseconds and it is impossible to use it for delays that are much shorter than 10 nanoseconds.
A known method for making an optical correlation for very small delays of about 100 picoseconds uses a device comprising:
a first laser diode and a second laser diode emitting on a same frequency and with an amplitude modulated respectively by the two signals to be correlated; PA1 an optical guide made of a LiNbO.sub.3 crystal, the two ends of this guide being coupled respectively to two laser diodes; PA1 a photomultiplier to collect the second harmonic emitted by the crystal, the intensity of this second harmonic expressing the correlation between the two signals modulating the light from the two laser diodes respectively. PA1 a first laser and a second laser emitting with a same frequency and with an amplitude modulated respectively by a first signal and a second signal to be correlated; PA1 an optic fiber with its two ends respectively coupled to the two lasers and having a length which is at least equal to the length corresponding to the maximum duration of the signals to be correlated, said optic fiber comprising, throughout its length, a part with its diameter reduced to let through an evanescent wave associated with the guided mode; PA1 a layer of non-centrosymmetrical type material in contact with the part having a reduced diameter, throughout the length of the fiber; PA1 means to collect the light emitted by the layer at a frequency which is twice the frequency of two lasers, and to give an electrical signal which is a function of the intensity of said light, this electrical signal expressing the correlation integral of the two signals to be correlated.
A device of this type is described by R. NORMANDIN and G. I. STEGEMAN in "Picosecond Signal Processing with Planar Nonlinear Integrated Optics", Appl. Phys. Lett. 36(4), 15 February 1980, p. 253, and by P. J. VELLA, R. NORMANDIN, G. I. STEGEMAN in "Enhanced Second Harmonic Generation by Counter-Propagating Guided Optical Waves", Appl. Phys. Lett 38(10), 15th May 1981, p. 759.
A LiNbO.sub.3 monocrystal cannot be more than a few centimeters long. It enables the envisaging of correlation between two signals for delays of smaller than 1 nanosecond or even in the range of one picosecond, but is not suitable in the 1-10 nanosecond range because the length of the crystal that can be made is not sufficient to be equal to the length corresponding to the propagation time of a pulse when this time is greater than or equal to one nanosecond.
The aim of the invention is to make a correlator which can be used to correlate two signals consisting of pulses having a duration of about 1 to 10 nanoseconds. The object of the invention is a correlator comprising an optic fiber in which two light signals expressing the two electrical signals to be correlated are propagated in opposite directions, said optic fiber comprising a flattened surface in contact with the surface of a thin layer of nonlinear material to emit a second harmonic, the amplitude of which expresses the correlation integral of the two signals.